1. Field of the Invention
The present invention relates to a WDM (wavelength-division multiplexing) optical communication technology and particularly it relates to a technology for controlling an optical tunable filter needed to extract signal light with a specific wavelength from a plurality of segments of signal light multiplexed by a WDM method, each with a different wavelength.
2. Description of the Related Art
With the explosive increase of a data communication demand centered on Internet traffic, a large capacity and super-long haul backbone network is required. Since in that case, a variety of user services are required, the realization of a high-reliable, flexible and economical network is also required.
By the progress of a wavelength-division multiplexing transmission technology and an optical amplification technology, recently transmission capacity and transmission distance have been remarkably increased and transmission line cost has also been reduced. However, if a conventional optical/electric conversion system or a electric switching system is adopted in order to follow a high-speed and large capacity signal and to increase the information processing capability of a network node, a node cost increases and the size of a node device also increases. In such a background, the development of an optical add drop multiplexer (OADM) and an optical cross connect (OXC) device, which replaces a large-scale electronic circuit with an optical device in an optical communication system in order to reduce the cost and size of a node and performs a variety of processes of data in units of an optical path in an optical wavelength area, is expected.
In these devices, a lot of optical function devices, such as an optical switch for turning signal light on/off, attenuating signal light and performing the 1×n switching of signal light, etc., a wavelength filter for distributing signal light for each wavelength and the like are used.
Of these optical function devices, an optical wavelength selecting device (hereinafter in this specification, called an “optical tunable filter”) that can select signal light with a desired wavelength from a WDM signal is a major key device for realizing an OADM. Such an optical tunable filter includes an acousto-optic tunable filter (AOTF).
FIG. 1 shows a network configuration of an OADM node, and FIG. 2 shows a configuration of an OADM using an AOTF.
In FIG. 1, a network A, 1001 and a network B, 1002 are overlapped in node 1, and the OADM of node 1 drops three segments of signal light each with one of wavelengths λ1, λ4 and λ6 of WDM signal light consisting of six segments of signal light each with one of wavelengths λ1 through λ6 transmitted from the node n in network A, 1001, from network A, 1001 and transmits them to the node 2 of network B, 1002. Three segments of signal light each with one of wavelengths λ2, λ3 and λ5 are transmitted through the OADM to the node 2 of network A, 1001.
The network shown in FIG. 2 is a ring network. The network comprises a running system (working system, indicated as system W), which is actually used, and a stand-by system (protection system, indicated as system P), which is used when system W fails. Since the respective configurations of systems W and P are the same, only the configuration of system W is described here. Although system W shown in FIG. 2 comprises three OADMs of OADM 1(W), OADM 2 (W) and OADM 3 (W), they have the same configuration. Therefore, only the configuration of OADM 1 (W) is described here. An amplified spontaneous emission (ASE) suppression filter 2000 inserted in the middle of system W or P eliminates natural light noise (white noise) generated and accumulated by each amplifier existing in the ring network.
WDM signal light transmitted to OADM1 (W) from OADM3 (W) is amplified up to a predetermined size by an amplifier 2001, and then is inputted to a photo-coupler (CPL) 2002. Although the signal light transmitted through the CPL 2002 is inputted to a rejection AOTF 2003, part of signal light demultiplexed by the CPL 2002 is inputted to an amplifier 2004. The signal light amplified by the amplifier 2004 is demultiplexed into a plurality of segments of signal light by a CPL 2005, and each demultiplexed are inputted to a drop AOTF 2006. The drop AOTF 2006 extracts signal light with a desired wavelength from the WDM signal light. The extracted signal light is designated as the drop output of the OADM 1 (W).
Then, this signal light from this OADM 1(W) is transmitted through an optical switch (OSW) 2100 for switching a system from W to P or vice versa, is inputted to a transponder 2200 and is demodulated.
The signal (ADD input signal) transmitted to the network from this node is, firstly, optically modulated by a tunable transponder 2300 to signal light with a predetermined wavelength. Then, the signal is inputted to the OADM 1 (W) through an OSW 2400. This signal light is multiplexed with signal light with a different wavelength transmitted from the node by a CPL 2007. The signal light multiplexed by the CPL 2007 is amplified up to a predetermined size by an amplifier 2008 and is inputted to a CPL 2009. Then, the signal light is inserted in the signal light transmitted without being blocked by the rejection AOTF 2003 of the signal light from the OADM 3 (W), and both are multiplexed. The signal light multiplexed by the CPL 2009 is amplified up to a predetermined size by an amplifier 2010 and then is transmitted toward the OADM 2 (W).
The signal light wavelength selecting characteristic of the rejection AOFT 2003 and the drop AOTF 2006 is controlled by a control unit (MC) 2011, based on information provided by a monitoring control system 3000 monitoring the operation of the entire network shown in FIG. 2.
As described above, an OADM node must have a function to insert (ADD) signal light with a desired wavelength in light that is transmitted through a node, a function to drop (extract) signal light with a desired wavelength from light that is transmitted through a node and a function to block signal light transmitted through a node. A function to collectively drop signal light sometimes is needed. This function is required in a node where two or more ring networks or general networks are overlapped, and is used to transmit signal light consisting of a plurality of segments of signal light each with a different wavelength from one network to another network. A function to collectively block signal light is also sometimes needed. This function blocks signal light with a wavelength to be terminated of light transmitting through a node and a plurality of segments of signal light each with a wavelength that has the same wavelength element.
In an OADM node, it is important for signal light with an arbitrary wavelength to be able to be demultiplexed and inserted for the purpose of the flexible operation of a network. For that purpose, it is necessary to apply the above-mentioned collective process to signal light with an arbitrary wavelength, and from this point of view, a device, such as an AOTF having a function to freely change a wavelength to be selected is useful. If signal light with a desired wavelength is selected and demultiplexed using this function to freely change a wavelength to be selected, the transmission center of the filtering characteristic of such a device must completely coincide with the desired wavelength of signal light. If the transmission center wavelength does not coincide with a signal light wavelength, for example, in the drop (extraction) process, insertion loss increases or signal light with another wavelength is wrongly dropped, which is the fatal problem of an OADM device.
Generally, the wavelength of light emitted from a laser diode (LD), being a transmitting light source, fluctuates, and the transmission center wavelength of such a device that provides a filter characteristic also fluctuates due to a change with an elapse of time, an environmental change, a control error and the like. Therefore, in order to stabilize the operation of an OADM device, it is indispensable to detect a wavelength fluctuation error and to track it to perform feedback control. In this tracking, in the case of a drop process, dropped signal light is demultiplexed and designated as monitor light, the monitor light is detected and its power value is controlled in such a way as to become a maximum. Usually, for example, a method for controlling by checking only the size of the receiving optical power of the monitor light, which is disclosed by Japanese Patent Laid-open No. 8-288932, is most economical and efficient.
However, since an AOTF has a selected wavelength fluctuation characteristic that is sensitive to ambient temperature, for example, a temperature change of only 1° C. leads to a100 GHz difference in a selected wavelength, the optimal frequency of an RF (high frequency) signal to be applied to determine a wavelength to be selected is not uniquely related to the wavelength to be selected and a wavelength to be selected varies due to fluctuations in ambient temperature. For example, although an AOTF selects a wavelength of 1,550 nm if an RF signal of 170 MHz is applied at ambient temperature of 25° C., it selects a wavelength of 1,558 nm if the same RF signal of 170 MHz is applied at ambient temperature of 35° C. Since the optimal frequency of an RF signal to be applied to an AOTF varies depending on ambient temperature, the AOTF sometimes wrongly selects signal light with a different wavelength in the selection of a signal with one arbitrary wavelength from a WDM signal.
If an optical power value supplied to a light detection unit detecting monitor light is small, the amount of fluctuation in a signal to be detected for tracking control also becomes small. Therefore, it is difficult to control tracking. Furthermore, the fact that since the amplitude of a signal to be detected becomes relatively small when setting in advance a wide dynamic range to avoid the saturation states of a photo-diode and an amplifier, an S/N (signal-to-noise) ratio degrades, the degradation of detection accuracy due to the fluctuations of another wavelength close to a dithering frequency and the like also become factors for difficult tracking control.
Regarding the above-mentioned problem, the applicant of the present invention has also previously applied for a patent with a method providing the light detection unit with a logarithmic amplifier, as shown in FIGS. 3A and 3B, to the Japanese Patent Office (Japanese Patent Application No. 2002-149555). This method is briefly described below.
FIG. 3A shows the configuration of a light detection circuit using a logarithmic amplifier.
In FIG. 3A, the current value of a monitor optical signal is converted into a size corresponding to its optical power value by a photo-diode (PD) 4001. This current value is inputted to the current/voltage conversion logarithmic amplifier 4002 which has the input/output characteristic shown in FIG. 3B, and is converted into a logarithmic voltage value. Then, this voltage value is amplified by a non-inversion amplifier 4003. Then, the high-wavelength element of the voltage value is eliminated by transmitting the value through a low-pass filter (LPF) 4004, and is inputted to an A/D converter 4005. Then, a digital signal corresponding to an input voltage value is outputted. This digital signal is transmitted to a driving circuit generating a RF signal to be applied to the AOTF as information for determining the optimal frequency of the RF signal.
In tracking control, the change of a transmitting optical power is observed and controlled by slightly changing a wavelength to be selected in the AOTF. However, if a logarithmic amplifier is used, as shown in FIG. 3B, a control signal is observed to change at a constant ratio against an input power in a logarithmic scale. Therefore, it can be easily controlled.
If the above-mentioned method for providing the light detection unit detecting monitor light with a logarithmic amplifier is adopted for the tracking control of the AOTF, the logarithmic amplifier outputs a voltage in a logarithmic scale. Therefore, a change in a large input signal is observed as a small change and a change in a small input signal is observed as a large change. Therefore, even a change in a very little optical power area that can never be observed can be observed. More specifically, for example, if there is a change of 10 dB in an input power area of −40 dBm, which cannot be signal light at all, even the output value of nearly zero measured in the light detection unit, not adopting a logarithmic amplifier that slightly changes, can be observed as it is in the light detection unit using a logarithmic amplifier. Therefore, such very little signal light is sometimes wrongly recognized as an actual WDM input signal.
In reality, since in a WDM system, a gain tilt is generated by the wavelength dependence of the amplification factor of an optical amplifier, a technology called “pre-emphasis” for transmitting a transmitting power after giving a tilt the reversal of that of the optical amplifier to it on a transmitting side is used. In a very high-speed and long-haul transmission system, since a gain tilt compensator is provided, a wavelength-transmitting power spectrum characteristic can be freely set. In other words, since there is a system in which the power spectrum of a WDM signal is not uniform and varies depending on a wavelength, for example, it is difficult to distinguish a very small change in a WDM signal composed of the remaining elements of a WDM signal obtained after it is cut and removed by an AWG (arrayed waveguide grating) or a filter from a WDM signal to which a tilt is given. Therefore, when selecting one segment of signal light with an arbitrary wavelength from the WDM signal, channel signal light with a different wavelength is sometimes wrongly selected.